React is a framework for blatantly obvious reasons.

It introduces JSX, which is technically speaking its own programming language independent of JavaScript.

It defines hooks like useState() and useContext() for state management, meaning it is not UI only, plus "function based" components that act as a pseudo DSL that is pretending to be functional JavaScript (which it isn't).

Most of the time you're expected to render the entire page body via react.

You couldn't get further away from the idea of a library.